Warhammer 40k painting tips and techniques. If I say what I want can people help me?

I need warhammer 40k painting tips and techniques? I am making an imperial fists apocalypse army, ultramarines, white scars and another chapter not sure what plus a death korp apocalypse army. I want to make my armies look like they have been fighting for more than 9 years on a forge world with 9 years worth of damages, bullet holes in armour, grime, oil scratches and martian dust on all of my army can any one help me? I also want to recreate the engine burn and worn down ramps and greasy look of the guns as well as on the thunderhawk gunship on forge world. I'm not sure if you can include photos on this site. Can anybody give me advice?

That's a long list of stuff!

For bullet holes and scratches, it's worth just scratching and drilling into the mini with a hobby drill and knife and (if you're on citadel paints) Nuln Oil into the holes.

For the others paint the model clean first then add the following:

Dust and mud are easy enough. Drybrush Dryad Bark (Dirt), Rhinox Hide (Mud), Ryza Rust/Kindleflame (Martian Dirt). Or little dabs of Agrax Earthshade depending on the mini. Start from the bottom and brush up. Adding more layers getting lower if needs be. Make sure to match colours with the base as far as possible.

For Oil stains, wash Serphim Sepia from the end of the pipe/barrel however much up you want the stain then Nuln Oil about 3/4 up that.

Less is best with these. You can always add more later.

Good luck and happy hobbying!